1. Technical Field
This invention relates generally to solid state pressure sensors and more particularly, to means and methods for protecting a pressure sensor element from the process media whose pressure is being sensed.
2. Background Art
Solid state pressure sensors are being employed in a variety of new applications because of their small size and compatibility with other electronic systems. Semiconductor chips or dice are frequently used as the pressure sensing elements. However, such semiconductor sensing elements are particularly sensitive to contamination. Thus, if the surface of the semiconductor sensing element is exposed directly to the process media whose pressure is being measured, the semiconductor sensing element may be adversely affected. The gel generally must remain insulative; contamination may render it conductive.
A number of different approaches have been used in the prior art in order to isolate the semiconductor sensing element from the process media being measured. Among these are, for example, the use of die coats and the use of metal diaphragms coupled to the semiconductor sensing element by rigid bars or levers. None of these approaches has proven entirely satisfactory. Accordingly, a need continues to exist for improved means and methods for protecting semiconductor pressure sensing elements from the process media whose pressure is being measured.
Another method includes providing a gel-like pressure transfer medium between the pressure sensing element and a prefabricated fluorosilicone diaphragm, such as is disclosed in U.S. Pat. No. 4,686,764.
The '764 method cannot provide highly reliable production of diaphragms which are flush with the face surface of the sensor body. The '764 method does not provide reliable initial pressurization sensitivity in sensors. The '764 method provides sensors the sensitivity of which may vary over time. The '764 method is time consuming and costly to make, and unreliable compared with the present invention. In contrast, the present invention is also economical, easy to manufacture, and lends itself to automated manufacture.
Accordingly, it is an object of the present invention to provide an improved means and method for protecting a semiconductor pressure sensing element from the process media whose pressure is being measured.
It is a further object of the present invention to provide an improved means and method whereby the semiconductor sensing element is protected from the process media without substantial loss in sensitivity.
It is an additional object of the present invention to provide an improved means and method for protecting a semiconductor pressure sensing element from the process media without the use of metal diaphragms with rods or levers.
It is a further object of the present invention to provide an improved means and method for protecting a semiconductor pressure sensing element which is of light weight and is an extremely shock resistant assembly.
Another object of the invention is improved product fabrication, product quality, and cost savings.
Yet another object of the invention is increased sensor accuracy due to a diaphragm having a low spring rate and a diaphragm which does not induce pressure on the sensor in connection with minimal volume gel fill.
It is an object of the present invention to enable very small diaphragm areas (by use of low modulus "rubber") which permits small (TO-8) sensor packaging.
Yet another object of the present invention is to provide sensors suitable for medical applications by the use of medical grade diaphragm materials.
Another object of the invention is reduced thermal error due to a diaphragm having a low spring rate and a diaphragm which does not induce pressure on the sensor element, in connection with minimal volume gel fill.
A significant manufacturing, quality, and performance advantage inheres in the method of fabrication, in that before the pressure transfer medium and the diaphragm are added to the sensor, the mounted sensor element or die may be tested and laser trimmed to the desired calibration. The initial flowable state of the pressure transfer medium and the diaphragm material allows the diaphragm to be applied in a zero stress state without inducing any force on the sensor. Significantly higher yields of tighter specification products result at lower cost.
Another advantage of the present invention is the ability of the pour-in-place diaphragm procedure to smoothly cover irregularities in the pressure transfer medium without creating air bubbles or pockets which could adversely affect performance.
Other advantages of the invention include: small size, sensors having compliant diaphragms which have a low spring rate; manufacturing ease; resistance to a broad spectrum of process media; low cost; enables sensing pressure applied by solid and/or semi-solid contact, not just gas or liquid; and the product design lends itself to inert plastic packaging.